Most of the info I can find on the phrase “leader of the free world” agrees that the term came about during the Cold War to describe the US president. I haven’t seen credit for a specific origin of the phrase, however, or an earliest cite. Does its use as a common phrase predate the Cold War? Has it always attached itself to the president, or have there been other world leaders so described before that office?

I’m similarly curious about “the most powerful office in the world” re: the presidency, or “the most powerful man in the world” re: the president.

newspaperarchive has the earliest citation as September of 1949 in reference to the US under Truman.

As a result of these efforts, policy papers have now been prepared for submission to the National Security Council and the President, probably shortly after Truman’s return from Key West. And it can be confidently reported that all the studies have pointed in the same direction—a much greater effort in all fields is required if the United States is not to fail in its role of leader of the free world.

Many other citations related to the President of the US. This citation is especially used as over against the Soviet Union. No surprise there.